Rename all?, any? and empty? (#6464)

Rename `all?`, `any?` and `empty?` to `all`, `any` and `is-empty` for sake of simplicity and consistency.

- More understandable for newcomers, that these commands are no special to others.
- `?` syntax did not really aprove readability. For me it made it worse.
- We can reserve `?` syntax for any other nushell feature.

crates/nu-command/tests/commands/empty.rs

@@ -11,8 +11,8 @@ fn reports_emptiness() {
                      [([[check]; [{}] ])]
             ]
             | get are_empty
-            | all? {
-              empty? check
+            | all {
+              is-empty check
             }
         "#
     ));